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River Flow Into Chesapeake Bay

During the 2015 water year (October 2014 to September 2015), river flow to the Chesapeake Bay averaged 41 billion gallons per day. This is below the normal range of flow, which indicates dry conditions.


Each day, billions of gallons of fresh water flow from rivers and streams into the Chesapeake Bay. The amount of water that flows out of these tributaries has a direct impact on pollution: as snowmelt or rainfall increase river flow, more pollution is pushed into the estuary. In other words, precipitation and river flow are two factors that affect pollution loads and water quality. River flow can also affect the Bay’s salinity and stratification.

Long-term trend (1937-2015)

Between 1937 and 2015, annual river flow to the Chesapeake Bay averaged 51 billion gallons per day. While it has ranged from 29.3 to 78 billion gallons per day, the normal range of flow for this time is between 43 billion and 57 billion gallons per day.

Analyses from the U.S. Geological Survey (USGS) indicate normal river flow prior to 1960, a dry period during the 1960s and wetter conditions in the 1970s. Since 1990, river flow has been extremely variable.

Short-term trend (2006-2015)

The last decade has seen variable river flow. While river flow during most water years—including 2006, 2007, 2008, 2012, 2013 and 2014—was relatively normal, flow during 2009 and 2015 was dry and flow during 2011 was wet. Indeed, water year 2011 was one of the five wettest years on record, due to a wet spring, a late summer hurricane and an early fall tropical storm.

Change from previous year (2014-2015)

Between 2014 and 2015, average river flow fell from 52.5 billion gallons per day to 41 billion gallons per day.

Additional Information

About half of the water in the Chesapeake Bay comes from its rivers and streams; the rest comes from the Atlantic Ocean. In an average year, three rivers deliver most (about 81 percent) of the river flow to the Bay: the Susquehanna (48 percent), the Potomac (19 percent) and the James (14 percent). The U.S. Geological Survey (USGS) uses monitoring data from the Susquehanna, Potomac and James to estimate river flow to the Bay. The USGS updates flow each month and compiles a summary each year. Trend analyses are available in USGS Circular 1316, Chapter 5 and in a more recent report assessing trends between 1985 and 2010.

Source of Data

US Geological Survey

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